Electrostatic seat occupant detection system

ABSTRACT

An electrostatic seat occupant detecting system of a vehicle has a vehicle body and a seat. The electrostatic seat occupant detecting system includes an antenna unit that includes a main electrode and a sub-electrode disposed near the main electrode, an AC power source, a switch unit for switching supply of AC voltage to the antenna unit to form one of a first electric field between the vehicle body and the main electrode thereby providing a seat-occupant detecting mode and a second electric field between the main electrode and the sub-electrode thereby providing a wet condition detecting mode, and a seat-occupant detecting ECU. The above seat-occupant detecting ECU includes an admittance calculating section that calculates a first admittance of the first electric field and a second admittance of the second electric field, a conductance susceptance separating section that separates conductance of the first and second admittance from the susceptance of the first and second admittances, a water content estimating section that estimates an amount of water content of the seat based on the conductance and the susceptance of the second admittance, and a judging section that judges whether a seat occupant is present or not based on one of the conductance and susceptance of the first admittance and the estimated amount of the water content of the seat.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority from JapanesePatent Application 2007-329257, filed Dec. 20, 2007, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a seat occupant detection system of avehicle that is used in a passenger protection system.

2. Description of the Related Art

JP-A-11-271463 discloses an electrostatic seat occupant detection systemthat employs a capacitance sensor and a passenger detecting ECU. Thecapacitance sensor provides an output signal as an amount of current ora level of voltage when an electric field between a main electrode and avehicle body changes.

However, such an occupant detection system may not operate accurately ifa seat of a vehicle gets wet with water because the state of theelectric field is considerably affected by water.

JP-A-2006-27591 or its counterpart US 2005/0275202 A1 discloses anotheroccupant detection system that warns a driver an abnormality and carriesout an abnormality process when a seat gets wet. However, it is notpossible for this system to accurately detect a seat occupant if a seatof a vehicle gets wet.

SUMMARY OF THE INVENTION

Therefore, an object of the invention is to provide an improved occupantdetection system that can accurately detect a seat occupant even if aseat of a vehicle gets wet.

According to a feature of the invention, an electrostatic seat occupantdetecting system includes an antenna unit that includes a main electrodeand a sub-electrode disposed side by side, an electric power source forsupplying AC voltage to the antenna unit, a switch unit for switchingsupply of the AC voltage to the antenna unit to selectively form a firstelectric field between the vehicle body and the main electrode therebyproviding a seat-occupant detecting mode or a second electric fieldbetween the main electrode and the sub-electrode thereby providing a wetcondition detecting mode, and a seat-occupant detecting ECU thatincludes an admittance calculating section that calculates a firstadmittance of the first electric field and a second admittance of thesecond electric field, a conductance-susceptance separating section thatseparates conductance of the first and second admittance from thesusceptance of the first and second admittances, a water contentestimating section that estimates an amount of water content of the seatbased on the conductance and the susceptance of the second admittance,and a judging section that judges whether a seat occupant is present ornot based on one of the conductance and susceptance of the firstadmittance and the estimated amount of the water content of the seat.

According to the above feature of the invention, a seat occupant can beaccurately detected even if the seat gets wet with fresh water or saltwater.

In the above electrostatic seat occupant detection system, assumingthat: the amount of water content is L; the conductance of the secondadmittance is Re; and the susceptance of the second admittance is Im,the amount of water content L is calculated according to the followingexpression: L=(Re²+Im²)^(1/2). The antenna unit may further includes aguard electrode disposed between a seat-cushion frame and the mainelectrode so as to confront the main electrode, and the guard electrodehas the same potential level as the main electrode. The judging sectionmay provide a threshold line that is a function of one of theconductance and the susceptance of the first admittance and judges thatan adult is seated on the seat when the amount of water contentestimated by the water content estimating section falls below thethreshold line. The judging section judges whether a seat occupant ispresent or not based on the conductance of the first admittance andprovides a threshold plane that is a function of the conductance and thesusceptance of the first admittance. The judging section judges that anadult is seated on the seat when the amount of water content estimatedby the water content estimating section falls below the threshold plane.

According to another feature of the invention, a passenger protectionsystem includes the electrostatic seat occupant detection system asstated above, an airbag ECU and an airbag unit. In the passengerprotection system, the airbag ECU operates the airbag unit according tothe result of judgment by the judging section of the above statedoccupant detection system.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and characteristics of the present invention aswell as the functions of related parts of the present invention willbecome clear from a study of the following detailed description, theappended claims and the drawings. In the drawings:

FIG. 1 is a schematic diagram illustrating an electrostatic seatoccupant detection system according to a preferred embodiment of theinvention;

FIG. 2 is a block diagram of the seat occupant detection systemaccording to the preferred embodiment of the invention;

FIG. 3 is a schematic diagram illustrating an antenna 2;

FIG. 4 is a cross-sectional view of the antenna illustrated in FIG. 3cut along line IV-IV;

FIG. 5 is a graph showing an Im-Re characteristic of an admittance;

FIG. 6 is a flow diagram of operation of the electrostatic seat occupantdetection system;

FIG. 7 is a graph showing an Im-Re characteristic of a first admittance;

FIG. 8 is a graph showing an Im-Re characteristic of a secondadmittance;

FIG. 9 is a graph showing an Re-Im-Le characteristic;

FIG. 10 is a graph showing an Im-L characteristic; and

FIG. 11 is a graph showing an Re-L characteristic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A vehicle occupant detection system according to a preferred embodimentof the present invention will be described with reference to theappended drawings.

A passenger protection system 100 is shown in FIG. 1. The passengerprotection system 100 includes an electrostatic seat occupant detectionsystem 1, a communication interface 15, an airbag ECU 16, and an airbagunit 17.

As shown in FIG. 2, the seat occupant detection system 1 includes anantenna 2, a vehicle seat 9, a connecting cable 13, and an occupantdetecting ECU 14. The antenna 2, the vehicle seat 9 and the cable 13form an occupant detecting circuit. The vehicle seat 9 includes a seatcushion 91 in which a seat-cushion frame 91 a is disposed and a seatback 92 in which a seat-back frame 92 a is disposed. The seat-cushionframe 91 a and the seat-back frame 92 a are electrically connected witha vehicle body 12 to provide a body earth or a ground level.

The antenna 2 is disposed between a seat cover and a cushion member ofthe seat-cushion 91 so as to confront the seat-cushion frame 91 a. Theantenna 2 includes a main electrode 21 and a sub electrode 22 which aredisposed on the seat-cover side and a guard electrode 23 which isdisposed on the cushion-member side.

The main electrode 21 and sub electrode 22 are disposed side by side ata prescribed distance, and the guard electrode 23 is disposed betweenthe main electrode 21 and the seat-cushion frame 91 a so as to confrontthe main electrode 21. The connecting cable 13 may be a wire harness orthe like for connecting the seat occupant detecting ECU 14.

The seat occupant detecting ECU 14 includes an electric power source 3,a switch unit 4, an admittance calculating section 5, a real-imaginarycomponent separating section 6, a water content estimating section 7 anda judging section 8. The electric power source 3 includes an oscillatingcircuit that provides alternating voltage of a fixed frequency.

As shown in FIG. 2, the switch unit 4 includes a movable contact 41 anda pair of stationary contacts 4 a, 4 b.

When the movable contact 41 is brought in contact with the stationarycontact 4 a, the main and sub electrodes 21, 22 are connected with theelectric power source 3 to provide a seat-occupant detecting mode.Therefore, electric fields are formed between the main electrode 21 andthe vehicle body 12 that includes the seat-cushion frame 91 a and theseat-back frame 92 a, and between the sub electrode 22 and the vehiclebody 12. In the seat occupant detecting mode, a bias voltage is appliedto the guard electrode 23 via an operational amplifier or the like fromthe power source 3 to put the guard electrode 23 to the same potentiallevel as the main electrode 21.

When the movable contact 41 is brought in contact with the otherstationary contact 4 b, the main electrode 21 and the guard electrode 23are body-earthed or grounded, and only the sub electrode 22 is connectedwith the electric power source 3. Accordingly, an electric field isformed between the sub electrode 22 and the main electrode 21 to providea wet condition detecting mode.

The admittance calculating section 5 is connected with the electricpower source 3, the switch unit 4 and the real-imaginary componentseparating section 6. The admittance calculating section 5 includes avoltage detecting circuit 51, a current detecting circuit 52 and aZ-value (or an impedance) calculating section 53. The voltage detectingcircuit 51 is connected with the electric power source 3 to detect theamplitude of alternating voltage applied across the electric powersource and the switch unit 4, and the current detecting circuit 52detects the amount of current flowing in a circuit formed between theelectric power source 3 and the switch unit 4.

The Z-value calculating section 53 is connected with the voltagedetecting circuit 51, the current detecting 52 and the real-imaginarycomponent separating section 6. The Z-value calculating section 53calculates an admittance (i.e. Y=1/Z) of the occupant detecting circuit,which includes the antenna 2, the seat 9 and the switch unit 4, from theamplitude of the voltage detected by the voltage detecting circuit 51and the amount of current detected by the current detecting circuit 52.When the seat occupant detecting mode is provided, the Z-valuecalculating section 53 calculates a first admittance Y₁. On the otherhand, the Z-value calculating section 53 calculates a second admittanceY₂ when the wet condition detecting mode is provided.

The first admittance changes as the impedance between a unit of the mainand sub electrodes 21, 22 and the vehicle body 12 changes, and thesecond admittance changes as the impedance between the sub electrode 22and the main electrode 21.

The real-imaginary component separating section 6 is connected with theadmittance calculating section 5, the water content estimating section 7and the judging section 8. The real-imaginary component separatingsection 6 calculates a conductance and a susceptance from the admittancethat is calculated by the admittance calculating section 5. In moredetail, the real-imaginary component separating section 6 calculates theconductance and the susceptance of the first admittance Y₁ when theoccupant detecting mode is provided and also the conductance and thesusceptance of the second admittance Y₂ when the wet condition detectingmode is provided.

The water content estimating section 7 is connected with thereal-imaginary separating section 6 and the judging section 8. The watercontent estimating section 7 estimates an amount of the water content ofthe seat based on the conductance and susceptance of the secondadmittance Y₂ that is calculated by the real-imaginary componentseparating section 6. The amount of the water content is represented bythe second admittance Y₂.

The judging section 8 is connected with the real-imaginary componentseparating section 6 and the water content estimating section 7. Thejudging section 8 judges whether a seat occupant is present or not basedon the conductance and susceptance of the first admittance Y₁ and theestimated water content of the seat.

The result of the judgment is sent via the communication interface 15 tothe airbag ECU 16. The airbag ECU 16 controls the airbag unit 17 basedon the result of the judgment. In other words, the airbag ECU 16determines whether to permit operation of the air bag unit 17 or not. Ifit is judged that the seat occupant is an adult, the operation ispermitted. In this case, the airbag ECU 16 operates the airbag unit 17if a collision is detected by an acceleration sensor (not shown). On theother hand, the airbag ECU 16 does not permit operation of the airbagunit 17 if it is judged that the seat is vacant or that the seatoccupant is a child or a child restraint system (CRS).

Incidentally, each of the Z-value calculating section 53, thereal-imaginary component separating section 6, the water contentestimating section 7 and the judging section 8 is constructed of aprocessing circuit or a processing program.

As shown in FIG. 3, the antenna 2 is a unit member that has generallyrectangular four antenna sections disposed side by side.

As shown in FIG. 4, each section of the antenna 2 is constructed of aportion of the main electrode 21, a portion of the sub electrode 22, aportion of the guard electrode 23, a portion of a base film layer 24, aportion of an upper film layer 25 and a portion of a lower film layer26.

The base film layer 24, the upper film layer 25 and the lower film layer26 are made of PET (polyethylene terephthalate) and are bonded to eachother by an adhesive agent 27.

The main electrode 21 is constructed of a carbon member 210 and a silvermember 211 and is disposed between the upper film layer 22 and the basefilm layer 24. The carbon member 210 has four electrode sections, eachof which is disposed at the central portion of each antenna section andis shaped into a rectangular plate. The silver member 211 is disposed inthe carbon member 210 near the peripheral portions thereof.

The sub electrode 22 is constructed of a carbon member 220 and a silvermember 221 and is disposed between the upper film layer 22 and the basefilm layer 24 to surround the main electrode 21. The carbon member 220also has four electrode sections, each of which is shaped into arectangular frame that surrounds one of the antenna sections, and thesilver member 221 is disposed in the carbon member 220 to extend alongthe center line thereof.

The guard electrode 23 is constructed of a carbon member 230 and asilver member 231. The guard electrode 23 is disposed between the basefilm layer 24 and the lower film layer 26 so as to confront the mainelectrode 21 via the base film layer 24 that is an insulating member.The carbon member 230 has four electrode sections, each of which isdisposed at the central portion of each antenna section to confront oneof the electrode sections of the main electrode 21 and is shaped into arectangular plate. The silver member 231 is disposed in the carbonmember 230 near the peripheral portions thereof.

The operation of the seat occupant detection system 1 will be explainedwith reference to a vector diagram shown in FIG. 5.

An impedance Z is expressed as follows.

Z=1/(Re+j·Im), in which: Re is a conductance; Im is a susceptance; and jindicates a unit of an imaginary number.

The length Y of vector Z is expressed as follows.

Y=(Re ² +Im ²)^(1/2)

At the first step S 101, the switch unit 4 connects the main electrode21 with the stationary contact 4 a so that an electric field between theunit of the main electrode 21 and the sub electrode 22 and the vehiclebody 12 can form, thereby providing the seat occupant detecting mode.

Then, the impedance Za and the first admittance Y₁ (which is 1/Za)between the unit of the main electrode 21 and the sub electrode 22 andthe vehicle body 12 are calculated by the admittance calculating section5 at step S102, where the conductance Re and the susceptance Im areseparated by the real-imaginary component separating section 6.

The first admittance changes according to the state of the seat as shownin a orthogonal coordinate system graph in FIG. 7.

In the case where the seat is dry (i.e. the water content of the seat iszero milliliter): when no adult or only a child restraint system (CRS)is seated, (Re, Im) of the first admittance is positioned at point P1near (6, 16) in the graph; and when an adult is seated, (Re, Im) ispositioned at P2 near (1, 63). Thus, the susceptance of the firstadmittance Y₁ when an adult is seated becomes much larger than thesusceptance when no adult is seated. Incidentally, the seat occupant canbe detected by examining if the value (Re, Im) is positioned above atwo-dot-chain line T or not in the case where the seat is dry.

In the case where the seat gets wet with fresh water: when no adult oronly a child restraint system (CRS) is seated, (Re, Im) of the firstadmittance Y₁ is positioned along arrow line A; and when an adult isseated, (Re, Im) of the first admittance Y₁ is positioned along arrowline B. It is noted that (Re, Im) of the first admittance does notlinearly increase when the water content becomes over 200 ml.

In the case where the seat gets wet with salt water: when no adult oronly a child restraint system (CRS) is seated, (Re, Im) of the firstadmittance Y₁ is positioned at point P3, which is near (5, 50) alongarrow line C; and when an adult is seated, (Re, Im) of the firstadmittance is positioned at a point near (7, 107), which is locatedalong arrow line D. It is noted that: the susceptance Im increases asthe content of salt water increases; and the point P3 is near point P2.Incidentally, the values (Re, Im) of the first admittance Y₁ in the casewhere the salt water content becomes about 100 ml are respectively (5,45) when no adult or only a child restraint system (CRS) is seated and(10, 100) when an adult is seated.

In the seat occupant detecting mode, the data (Re, Im) are sent to thejudging section 8. Subsequently, the switch unit 4 switches the movablecontact 41 from the stationary 4 a to the stationary contact 4 b toprovide the wet condition detecting mode at S103.

At the step S104, wet condition is detected according to characteristicsof the second admittance Y₂ in an orthogonal coordinate system graphshown in FIG. 8.

In the wet condition detecting mode, an electric field is formed betweenthe sub electrode 22 and the main electrode 21. The electric fieldchanges if a portion of the seat cushion 91 in the electric field getswet.

In the case where the seat is dry (or the water content of the seat iszero milliliter): (Re, Im) of the second admittance Y₂ is positioned ata position near (1, 30) in the graph, irrespective of whether an adultor CRS is seated or not.

In the case where the seat gets wet with fresh water of 100 ml: when noadult is seated, (Re, Im) of the second admittance Y₂ is positioned near(35, 60); when CRS is seated, (Re, Im) of the second admittance Y₂ ispositioned near (110, 125); and when an adult is seated, (Re, Im) of thesecond admittance Y₂ is positioned near (130, 125).

In the case where the seat gets wet with fresh water of 200 ml: when noadult is seated, (Re, Im) of the second admittance Y₂ is positioned near(100, 100); when CRS is seated, (Re, Im) of the second admittance Y₂ ispositioned near (195, 140); and when an adult is seated, (Re, Im) of thesecond admittance Y₂ is positioned near (235. 130).

In the case where the seat gets wet with salt water of 100 ml: when noadult is seated, (Re, Im) of the second admittance Y₂ is positioned near(75, 155); when CRS is seated, (Re, Im) of the second admittance Y₂ ispositioned near (125, 190); and when an adult is seated, (Re, Im) of thesecond admittance Y₂ is positioned near (150, 185).

In the case where the seat gets wet with salt water of 200 ml: when noadult is seated, (Re, Im) of the second admittance Y₂ is positioned near(110, 175); when CRS is seated, (Re, Im) of the second admittance Y₂ ispositioned near (190, 205); and when an adult is seated, (Re, Im) of thesecond admittance Y₂ is positioned near (245, 195).

Thus, as the content of fresh or salt water increases, the value (Re,Im) shifts to the right in FIG. 8. It is noted that the water contentcan be estimated by the distance of the value (Re, Im) from the originof the orthogonal coordinate system graph shown in FIG. 8.

The water content estimating section 7 estimates a water content Laccording to the value (Re, Im) of the second admittance Y₂. Here, thewater content L is expressed as follows: L=(Re²+Im²)^(1/2). That is, thewater content L is the same as the length of the second admittance Y₂.

The data of the water content L are sent to the judging section 8, andthe occupant detection is carried out at step S105 in a manner describedhereafter with reference to FIGS. 9-11.

The judging section 8 judges whether a seat occupant is present or notaccording to the value (Re, Im) of the second admittance Y₂ and thewater content L, which can be plotted in a three dimensional orthogonalcoordinate system graph shown in FIG. 9.

In FIG. 9, the point P2 that indicates that an adult is seated on a dryseat and the point P3 that indicates that no adult is seated on the seatthat is wetted with salt water are positioned close to each other.However, the water content L of the point P3 is much larger than thecontent of the point P2, as shown in FIG. 10, where various values ofthe conductance on a line E-E or of the same conductance Re are plotted.In other words, the points P3 and P2 can be easily judged by comparingthe contents L of the points P2 and P3 with a threshold line (dashedline) T_(IL).

When the susceptance Im is located in an area between the origin and apreset point G, the threshold value of the threshold line T_(IL) is 0.On the other hand, the threshold value of the threshold line T_(IL)increases as the susceptance Im increases.

If the water content L is smaller than the threshold line, it is judgedthat the susceptance Im is changed by an adult who is seated on theseat. For example, the water content L at the point P2, which indicatesthat an adult is seated on the seat that is dry, is smaller than thethreshold line T_(IL).

On the other hand, it is judged that no adult but CRS is seated on theseat, because the water content L at the point P1 is larger than thethreshold line T_(IL). Further, because the water content L at the pointP3 is larger than the threshold line T_(IL), it is judged that no adultbut CRS is seated on the seat that is wetted with salt water.

In the case where the seat is wetted with salt water, as the watercontent L increases the susceptance Im increases. However, the judgmentcan be made correctly because the threshold value increases as thesusceptance Im becomes larger.

Various values of the conductance Re on a dashed line F-F or of the samesusceptance shown in FIG. 9 are plotted in FIG. 11. It is understoodthat the threshold value increases along the threshold line T_(RL) asthe value of the conductance Re becomes larger. As indicated by thearrow line B in FIGS. 7 and 9, the value of the conductance Re increasesas the water content L increases, although the value of the susceptanceIm does not increase. In other words, the values of the conductance Reand the water content L increase as the content of the fresh waterincreases when the susceptance Im has a certain value. Thus, when anadult is seated on the seat that is wetted with fresh water, the watercontent L falls below the threshold line T_(RL).

The white (or blank) bar P4 in FIG. 11 indicates a value of the watercontent L when no adult but CRS is seated on a seat that is wetted withsalt water. That is, this value of the water content L is located at across point (not shown) of the extension of the arrow line C and thedashed line F-F in FIG. 9. It is understood that the water content Lthat is indicated by the white bar P4 is much higher than the thresholdline T_(RL) so that “vacant seat or CRS” can be clearly judged.

As shown in FIG. 9, the threshold line T_(RL) is included in a thresholdplane T_(RIL) that discriminates a seat occupied by an adult, which isindicated blacken (or solid) bars located below the threshold planeT_(RIL), from a vacant seat or seat with a CRS, which is indicated bywhite bars located above the threshold plane T_(RIL). The judgingsection 8 judges the state of the seat by comparing the value of Re, Imand L with the threshold plane T_(RIL). For example, the judging section8 judges that an adult is seated if the water content L, which isestimated by the water content estimating section 7, is smaller than avalue on the threshold plane T_(RIL) at the first admittance Y₁, whichis calculated by the real-imaginary component separating section 6.

The water content L can be calculated by using an orthogonal coordinategraph in which a vacant dry seat is set at the origin thereof.

Assuming that the second admittance (Re, Im) of the vacant dry seat is(Re₀, Im₀), the water content L is expressed as follows.

L={Re−Re ₀)²+(Im−Im ₀)²}^(1/2)

At step S106, the result of judgment is transmitted to the airbag ECU 16via a communication I/F. Thereafter, the steps 1-6 are repeated.

The airbag unit 17 is controlled according to the result of judgment.For example, if the result indicates that an adult is seated on a seat,the airbag ECU 16 permit operation of the airbag unit 17 so that theairbag can expand when a collision is detected. If the seat is vacant,or a CRS is seated thereon, the airbag ECU 16 forbids the operation ofthe airbag unit 17.

The passenger detection system 100 may include an alarm. If theelectrodes are short-circuited due to a much amount of water coveringthe seat, a driver or a passenger can recognize a trouble by the alarm.

In the foregoing description of the present invention, the invention hasbeen disclosed with reference to specific embodiments thereof. It will,however, be evident that various modifications and changes may be madeto the specific embodiments of the present invention without departingfrom the scope of the invention as set forth in the appended claims.

Accordingly, the description of the present invention is to be regardedin an illustrative, rather than a restrictive, sense.

1. An electrostatic seat occupant detecting system of a vehicle having avehicle body and a seat, the electrostatic seat occupant detectingsystem comprising: an antenna unit that includes a main electrode and asub-electrode disposed side by side; an electric power source forsupplying AC voltage to the antenna unit; a switch unit for switchingsupply of the AC voltage to the antenna unit to form one of a firstelectric field between the vehicle body and the main electrode therebyproviding a seat-occupant detecting mode and a second electric fieldbetween the main electrode and the sub-electrode thereby providing a wetcondition detecting mode; and a seat-occupant detecting ECU, wherein theseat-occupant detecting ECU comprises: an admittance calculating sectionthat calculates a first admittance of the first electric field and asecond admittance of the second electric field; aconductance-susceptance separating section that separates conductance ofthe first and second admittances from the susceptance of the first andsecond admittances; a water content estimating section that estimates anamount of water content of the seat based on the conductance and thesusceptance of the second admittance; and a judging section that judgeswhether a seat occupant is present or not based on one of theconductance and susceptance of the first admittance and the estimatedamount of the water content of the seat.
 2. An electrostatic seatoccupant detection system as in claim 1, wherein assuming that: theamount of water content is L; the conductance of the second admittanceis Re; and the susceptance of the second admittance is Im, the amount ofwater content L is calculated according to the following expression:L=(Re²+Im²)^(1/2).
 3. An electrostatic seat occupant detection system asin claim 1, wherein: the seat includes a seat frame; the antenna unitfurther includes a guard electrode disposed between the seat-cushionframe and main electrode so as to confront the main electrode; and theguard electrode is arranged to have the same potential level as the mainelectrode.
 4. An electrostatic seat occupant detection system as inclaim 1, wherein the judging section provides a threshold line that is afunction of one of the conductance and the susceptance of the firstadmittance and judges that an adult is seated on the seat when theamount of water content estimated by the water content estimatingsection falls below the threshold line.
 5. An electrostatic seatoccupant detection system as is claim 1, wherein the judging sectionjudges whether a seat occupant is present or not based on theconductance of the first admittance.
 6. An electrostatic seat occupantdetection system as in claim 1, wherein the judging section provides athreshold plane that is a function of the conductance and thesusceptance of the first admittance and judges that an adult is seatedon the seat when the amount of water content estimated by the watercontent estimating section falls below the threshold plane.
 7. Apassenger protection system comprising the electrostatic seat occupantdetection system as in claim 1, an airbag ECU and an airbag unit,wherein the airbag ECU operates the airbag unit according to the resultof judgment of the judging section.